Miso (味噌) is Japan’s fermented soybean paste: cooked soybeans mashed with koji – rice, barley or soybeans grown over with the mold Aspergillus oryzae – and salt, then packed into a crock and left for months. It seasons miso soup, marinades, dressings and glazes. Homemade miso, called temae miso, is a long-standing Japanese household tradition, and it’s one of the more forgiving long ferments to make yourself.
How Miso Works
Miso is made in two stages (Allwood et al., 2021). First, the koji mold is grown on grain to make koji, which is full of enzymes. Then the koji is mixed into a salted soybean mash, where over months – up to two years – those enzymes break starch into sugars and protein into amino acids (the savory, umami taste), while salt-tolerant bacteria and yeasts slowly add acidity and aroma. The longer and saltier the fermentation, the darker and stronger the miso.
Types of Miso
- By koji: rice miso (kome), barley miso (mugi) and soybean miso (mame, such as Nagoya’s dark hatcho-style miso).
- By color and age: sweet white miso has a high proportion of koji, less salt and a short fermentation; red miso has less koji, more salt and a long one.
- Salt: Japan’s food tables list sweet white miso at 6.1 g salt per 100 g and red rice miso at 13.0 g (MEXT; MEXT). Miso is a salty seasoning – a tablespoon of red miso holds roughly 2 g of salt.
Korean doenjang is a cousin made from blocks of fermented soybeans rather than koji; see how they compare in our fermented soybean guide. For a quick koji seasoning without soybeans, try shio koji.
Mold, Film and Tamari
Over months, the surface of homemade miso will change. Dark liquid (tamari) seeping up is normal – stir it back in or use it as a seasoning. A thin, flat white film is usually film yeast, which forms where miso touches air: lift it off and the miso below is fine. Fuzzy mold that’s green, blue, black, pink or orange is different. Mold can send roots into soft foods, and USDA advises discarding soft foods with mold rather than cutting it off (USDA). The best defence is prevention: enough salt, a tightly packed paste with no air pockets, a salted surface covered with film and a weight, and a cool spot.
Storing Miso
Once it tastes the way you like, move the miso to clean jars and refrigerate it; Japan’s largest miso maker recommends the fridge or freezer to keep its flavor (Marukome). Miso doesn’t freeze solid, so it can be used straight from the freezer. For how long each type keeps, see how long miso lasts.
In miso soup, stir the miso in after taking the pot off the heat: boiling dulls its aroma, and if your miso is unpasteurized, it also kills its live microbes.